The present invention relates generally to inkjet printers and, in particular, to an ink delivery system and method for controlling fluid pressure therein.
Many manufacturers of inkjet printers today expend considerable effort toward developing higher-performing and longer-lasting inkjet printheads. The typical inkjet printhead comprises a silicon substrate, structures built on the substrate, and connections to the substrate. Such a printhead typically uses liquid ink (i.e., dissolved colorants or pigments dispersed in a solvent). The printhead has an array of precisely formed orifices or nozzles attached to the substrate that incorporates an array of ink ejection chambers which receive liquid ink from an ink reservoir. Each chamber is located opposite the nozzle so ink can collect between it and the nozzle. The ejection of ink droplets is typically under the control of a microprocessor, the signals of which are conveyed by electrical traces to resistor elements on the substrate. When electric printing pulses heat a resistor element, a small portion of the ink next to it vaporizes and ejects a drop of ink from the printhead. Properly arranged nozzles form a dot matrix pattern. Properly sequencing the operation of each nozzle causes characters or images to be printed upon the paper as the printhead moves past the paper.
Unfortunately, the ability of traditional ink delivery systems to meet printhead thermal demands arising from higher drop firing frequencies, denser resistor spacing, larger die sizes, and decreased ejection efficiency is in doubt. In addition, current ink delivery systems struggle to manage air and particles in the printhead so that its long-term reliability is not compromised. These problems are compounded when supplying ink to an array of printheads.
Recirculating ink delivery systems have been proposed by many as a solution to these problems. While these systems are generally capable of removing heat, air and particles, they typically rely on passive hydrostatics (fluid column height relative to the printhead) to maintain appropriate backpressure at the printhead. xe2x80x9cBackpressurexe2x80x9d is the term used to describe what is typically a slightly negative pressure relative to atmospheric pressure at the printhead that prevents ink from leaking out of the printhead nozzles in between periods of active ink ejection. Care must be exercised in setting such backpressures. An overly large backpressure (i.e., an excessively negative pressure) will prevent ink from being drawn through the printhead, thereby xe2x80x9cstarvingxe2x80x9d the printhead of ink. An overly small backpressure (i.e., an insufficiently negative or even positive pressure) will cause too much ink to flow out of the printhead nozzles, thereby causing the printhead to xe2x80x9cdroolxe2x80x9d excess ink.
A system relying on passive hydrostatics to control backpressure is illustrated, for example, in U.S. Pat. No. 4,929,963. While these systems are effective, they appear to be generally limited to precisely-positioned arrangements that consume considerable space. In addition, backpressure adjustments done by reservoir positioning systems add to cost and are similarly space-consuming. Finally, pressure-priming of the ink delivery system and printhead typically requires pressurizing air above a fluid reservoir, resulting in lengthy startup and service times.
To overcome some of these shortcomings of the prior art, active-control ink delivery systems have been proposed. Generally, these systems have controlled backpressures by modulating ink pressures upstream of the printhead (i.e., in the ink supply side of the printhead). For example, U.S. Pat. No. 5,880,748 illustrates an actively controlled ink delivery system in which ink pressures upstream of the printhead are monitored and, when necessary, used to control a valve which affects the backpressure of the ink being delivered to the printhead. Likewise, U.S. Pat. No. 5,646,666 teaches the use of a pump and vacuum regulator for maintaining a partial vacuum, and hence a slight backpressure, at the ink reservoir supplying the printhead. The current state of the art with respect to active control of backpressures in ink delivery systems has not, however, addressed the possibilities for regulating backpressures through the active control of pressures downstream of the printhead (i.e., in the ink return side of the printhead). Thus, it would be advantageous to provide a an ink delivery system that incorporates active pressure control downstream of the printhead.
The present invention provides an actively-controlled recirculating ink delivery system that overcomes the shortcomings of prior art systems and incorporates active control of downstream pressures to control backpressure. Generally, this is achieved through the use of a device that provides active control, when needed, of downstream ink pressures. Such a device may comprise a pump, a return valve, combination thereof or other similar devices. The present invention also incorporates the use of a pressurized ink supply, pressure sensors, an air and heat exchanger, and other components such as a compliant element, filters, and thermocouples to further refine and improve performance of the ink delivery system. Because the pressurized ink supply is not restricted to sit at a particular vertical distance below the printhead, backpressure may be changed quickly and easily through electronic valve control, and system priming is considerably quicker than with conventional air-pressurized systems.